could she be pregnant

what are the odds of a vasectomy failing and having 6 negative pregnancy tests and actually being pregnant. I have had a vasectomy, i just took my last sperm analysis about 3 weeks ago. Both of my sperm tests were negative. My wife has missed her period 9 days now. She has taken 6 pregnancy tests all negative. We have called the doctor to do a pregnancy test but they said wait another 2 weeks to see if she has her period because the odds of her being pregnant are ridiculously low. And for some reason i just cant convince her she isnt pregnant like im getting so frustrated shes crying and stuff and im trying to explain to her.. you know what are the odds of a vasectomy failing and 6 pregnancy tests giving false negatives.. please help i dont know what to do or say anymore it is so frustrating.. and just a little additional info.. were in our 20's so no menopause.